1) No idea, don't remember seeing those.
2) Do you want the "profile | register | search" etc centered underneath it? If so, open up public_common.pl, and in the section "$standard_title_table = qq~ ", change line 167 from
to
and line 169 from
to
3) In the CP, in the Style Template associated with that page, enter the name of the file under "Page Background Graphic". Place the graphic itself in your non-cgi folder. Note that this will fill the page background but not the forum itself.